A printed circuit board (“PCB”) is a thin plate on which integrated circuits and other electronic components (e.g., discrete) are mounted. For example, computers typically include one or more PCBs such as motherboards, controller cards, and network interface cards (“NIC”). Once a PCB has been manufactured, it undergoes testing, including a continuity test to ensure that no opens or shorts exist on the conductive trace routes. PCBs can be quite large, containing intricate patterns (e.g., “layouts”) of hundreds of traces. Many high-density printed circuit products have been developed for High Density Interconnects (“HDI”) applications. Examples of HDI applications include integrated circuit packaging, hard disk drives, mobile phones, and flat panel displays. These applications require high-density circuits having fine lines smaller than 50-micron lines and spaces, and tight contact pads, which have a pitch smaller than 100 microns. However, the production of high-density circuits and other PCBs are not reliable, requiring electrical continuity tests to be performed on various trace routes of the circuit board.
One method for testing the continuity of a trace routes on circuit boards includes optically inspecting each trace for opens or shorts using, for example, automatic optical inspection (“AOI”). However, optical methods have not proved as reliable as physically testing the electrical connection of each circuit. Another method includes electrically testing the continuity of each trace. Electrical open/short tests (e.g., by introducing a charge or voltage to a location of a trace and detecting the introduced charge or voltage at other locations of the trace), which provide more reliable results than optical testing, have limitations of their own. Current electrical test equipment uses contact pin probes, but these probes have physical limitations in the pitch size of the probes. Typically, 0.3 mm pitch probes may be the lowest or best size resolution or spacing limit for testing a single pad line and 0.2 mm pitch probe arrays for dual line pads. In addition, the contact pin probes may cause mechanical damage and stains to the circuit during testing. In addition, high pressure may be required to make exact electrical contact on each pad of high-density circuits. As such, the probes may cause dents on a small pad which requires a uniform surface to make a reliable connection and which is fragile against mechanical force, especially for flying leads of flex circuits.
Charge coupled device (“CCD”) imagers have been used for testing the continuity of traces on a PCB. These imagers allow trace routes to be visually inspected. However, CCD imagers are severely restricted in their size (because of the limitations of producing large-scale wafers); thereby making them impractical for testing of large size PCBs.